The chemical reprocessing of spent nuclear reactor fuel elements to recover the unburned nuclear reactor fuel material generates large volumes of aqueous solutions containing radioactive wastes. In addition to the large volumes produced, the aqueous waste solutions are extremely corrosive and present difficult problems in their handling and storage. Since it is necessary to store these radioactive wastes for extremely long periods of time to permit decay of the highly radioactive fission products included in the wastes, the aqueous wastes are converted to a solid form which, in addition to occupying less volume than the corresponding liquid wastes, is less corrosive and poses less difficult problems in handling and long-term storage. These aqueous radioactive waste solutions can be converted to solid form by spray solidification, fluidized-bed calcination, pot calcination or by heating to dryness and sintering the resulting solid.
Many of these radioactive waste solutions contain substantial quantities of nitrates and nitrites, generally as sodium nitrate. The formation of solids by any of the aforementioned methods from waste solutions containing nitrates and nitrites results in the formation of large quantities of noxious NO.sub.x gases.
At the present time these noxious gases are released to the atmosphere along with the off-gas from the solidification process. This disposal method is obviously undesirable since the nitrous oxides are pollutants in their own right and act as initiators of complex photochemical reactions with hydrocarbons. Some attempts at control are being made such as by passing the off-gas through separators where the nitrous oxides are removed from the off-gas by sorption on liquids or solids, thermal reduction by burning in a fuel-rich flame or by vapor-phase reaction with other compounds. The off-gas may also be contacted with a catalyst which will reduce the nitrogen oxides with or without the addition of a reducing gas.
None of the above alternatives is completely satisfactory in that problems exist with any of the suggestions; for example, the sorption liquids or solids must be disposed of or recharged for further use, while catalysts have a tendency to become poisoned and lose their efficiency.